The present invention relates to a process for the production of foam cushions from different fluid reaction mixtures. The resultant cushion consists of at least two foams differing correspondingly in elasticity or rigidity. Preferred are polyurethane foams. In the process, a first and at least one other reaction mixture are introduced into a mold cavity and left to react to form a cushion having zones of different elasticity or rigidity before the cushion is removed from the mold.
The production of molded parts and in particular cushions, by foaming in molds is becoming more widely used over the previously employed method of cutting. The foaming process is relatively simple and can be carried out almost loss free. When this process of foaming in the mold is employed, the cushions are either subsequently covered with a textile cover or the cover is directly back-foamed in a vacuum mold.
Various processes have been proposed for the production of molded parts composed of zones of differing rigidity.
According to EP-B1-68 820 (corresponding to U.S. Pat. No. 4,714,574), a second foam mixture is directly introduced on a first foam mixture at a time corresponding to a volumetric expansion of the first mixture of 100% to 2,300% so that one or more relatively rigid zones are formed within a relatively flexible foam. For this procedure, it is necessary to wait for the first reaction mixture to attain the necessary volumetric expansion before the second reaction mixture may be introduced. This has the undesirable effect of increasing the length of the foaming cycle in the mold. Moreover, the main aim of covering the more rigid zones with a layer of flexible foam to increase the seating comfort is difficult to achieve because the expansion of the flexible layer is limited to only small thicknesses in certain zones. Difficulties are also encountered in making these coverings sufficiently reproducible.
According to another process (U.S. Pat. No. 4,190,697), a more rigid foam formulation is introduced into the mold and allowed to start foaming. A second more flexible foam formulation is introduced when the first mixture has expanded to 10 to 80% of its full volume. This second mixture penetrates the first mixture when it is introduced and causes the first mixture to float upwards. Both mixtures then undergo reaction to produce a polyurethane foam product which has zones of differing density. This process has the disadvantage that penetration of the first reaction mixture introduced and the flow of second reaction mixture under the first mixture take place in an irregular fashion so that the properties of the molded parts (particularly the indentation hardness) obtained are not sufficiently reproducible.
None of these known processes suggest how, in the production of highly contoured cushions such as seating cushions with raised side parts, it is possible to prevent the reaction mixture in the region of the flat central part of the seat from flowing down into the parts of the mold cavity which are to form the side parts. In practice problem has been solved by placing barriers at the bottom of the mold cavity to limit the flow of the reaction mixture which is to form the flat central part of the seat, so that cushions with zones of differing hardness can be obtained in a reproducible manner. Barriers of this kind naturally increase the cost of the mold and leave grooves, (so called "pipes") in the part of the cushion in which they are placed. These grooves are in many cases covered up to some extent by providing beading in the form of piping on the cover in these positions. These grooves are, however, weak points which form the starting points for cracks when subjected to excessive loads.
The problem therefore arose of providing a process by which foam cushions of the type defined above may be produced more reproducibly and more simply, and in particular with shorter operating cycles, and which would also be able to withstand exceptional loads.